Issue: #6501 Wrong segmentNumShift was calculated in copyWithNewRepresentation

In DefaultDashChunkSource.copyWithNewRepresentation, it will handle the logic that
new MPD manifest file is updated and calculate a newSegmentNumShift for furthermore
segNum index calculation in getSegmentUrl, when a shorter window MPD updated and then
back to a longer window MPD, copyWithNewRepresentation will go into the overlap case
but the new index actually contains the old index..

library/dash/src/main/java/com/google/android/exoplayer2/source/dash/DefaultDashChunkSource.java

@@ -700,6 +700,8 @@ public class DefaultDashChunkSource implements DashChunkSource {
         // There's a gap between the old index and the new one which means we've slipped behind the
         // live window and can't proceed.
         throw new BehindLiveWindowException();
+      } else if (oldIndex.getFirstSegmentNum() >= newIndexFirstSegmentNum) {
+        // The new index contains the old one, continue process the next segment
       } else {
         // The new index overlaps with the old one.
         newSegmentNumShift +=